KVM: LoongArch: selftests: Add basic PMU event counting test

KVM: LoongArch: selftests: Add basic PMU event counting test

Introduce a basic PMU test that verifies hardware event counting for
four performance counters. The test enables the events for CPU cycles,
instructions retired, branch instructions, and branch misses, runs a
fixed number of loops, and checks that the counter values fall within
expected ranges. It also validates that the host supports PMU and that
the VM feature is enabled.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>